Many conventional valve devices such as plug valves, ball valves, butterfly valves and control type valves can be made to perform as high rangeability, adjustable flow restrictors. To accomplish this the control element must be moved through its travel very precisely. Currently available valve positioning devices achieve accuracies of .+-.0.5% of full travel and even .+-.0.25% can be achieved. Such accuracies, however, are not sufficient to accomplish high rangeabilities of flow control. There are increasing requirements for flow rangeabilities which exceed 1000:1 and there is of course an attraction to utilizing conventional valves of proven design for such applications. There is, therefore, a need to position such valves more precisely. In order to achieve such rangeabilities it is necessary to control positioning to accuracies of .+-.0.25%. Such high degree of accuracy in positioning is required because the control element of a conventional valve is proportioned to allow high flow through a pipe sized passage. Conversely however, when the control element works close to the fully closed position, the slightest movement has a great effect on flow. The higher gain which is experienced in the low flow regions generally makes the conventional type valve unsatisfactory for control of low flow and therefore limits the control rangeability. If effective flow control is to be achieved in the zones of low flow the valve element must be moved in very small increments or steps and the capability for making such steps should not restrict the availability of full thrust or torque needed to move the valve element when subjected to maximum loads.